Geoelectrical investigation of old/abandoned, covered landfill sites in urban areas: model development with a genetic diagnosis approach

Abstract

Geoelectrical methods have an important, albeit difficult role to play in landfill investigations. In the present economic conditions, with the environmentally sensitive regime, adequate desk-study and model development are essential ingredients for a successful site investigation of landfills. This paper attempts to develop a genetic investigative model for old/abandoned landfill sites where the records of operations are not available. The main elements of the model are the site boundaries, age and nature of anthropogenic deposits, depth and dip of the layers of refuse and sealing materials, the integrity and shape of the capping zones or separating walls and basal floor slopes, the position of concealed access roads in the site, the water table (or perched water bodies within the refuse) and the presence of leachate. The attendant geotechnical, hydrogeological, and bio-geochemical constraints at such sites are also incorporated in the model for consistency of practical solutions to landfill problems. The nature of anthropogenic deposits and the spatial–temporal characteristics of leachates are reviewed in a geoelectrical context. The analogy between waste degradation and leaching, and the well-known weathering processes of supergene mineral enrichment and saprolite formation in crystalline rocks is explored, and used to develop a conceptual resistivity-vs.-depth model for landfill sites. The main tenet of the model is that vertical conductivity profiles will attain maximum values in the zone of mineral enrichment near the water table and tail-off away from it. This conceptual resistivity model is shown to be consistent with non-invasive observations in landfill sites in different geographical environments. Power–law relationships are found to exist between some geoelectrically important hydrochemical parameters (fluid conductivity, chloride content and total dissolved solids) in leachates and leachate-contaminated groundwater from some landfill sites. Since some chemical parameters of fill are known to vary consistently with time, a plausible hydrochemical and age-deductive scheme for saturated fill is proposed for geoelectrical models of landfills without significant amounts of metal. Practical suggestions are made for a consistent approach in geoelectrical investigation and diagnosis of old landfill sites. A few field examples are used to illustrate the diagnosis approach.